1. Field of the Invention
The present invention relates to mobile communication services, and more particularly to a method for transmitting and receiving short message broadcasting services in a code division multiple access communication system.
2. Description of the Related Art
Short Message Service (SMS) procedure for a code division multiple access (CDMA) system includes transmitting broadcast messages from a Short Message Service Center (SMSC) to mobile stations through a base station (BS), and receiving the broadcast messages transmitted from the BS at a mobile station MS. Generally, the BS transmits broadcast messages to a MS through two forward channels, namely a paging channel and a broadcast channel. A MS receiving the SMS chooses the messages from among the transmitted broadcast messages.
The BS may transmit broadcast messages using one of various transmitting methods such as a multi-slot broadcast transmission, a multi-slot broadcast paging or a periodic broadcast paging. Of these methods, the multi-slot broadcast message transmission is the simplest, causing the least amount of delay in transmission, and is used regardless of whether a periodic broadcast paging transmission is available. In the multi-slot broadcast message transmission, broadcast messages are transmitted from a BS to MSs within the service region of the BS through all slots of a control channel. However, because the broadcast messages are transmitted using all slots of the control channel, an overload of the control channel may occur, thereby impeding transmission of messages other than the broadcast messages.
On the other hand, the multi-slot broadcast paging is used when the periodic broadcast paging is unavailable. In this method, a general paging message of a relatively small size, which indicates a forthcoming transmission, is first transmitted to MSs through all slots of the control channel before transmitting a broadcast message of a large size.
However, the periodic broadcast paging is a more efficient method than the multi-slot broadcast transmission as well as the multi-slot broadcast paging methods. In the periodic broadcast paging, a BS gives a notice to MSs of the periodic transmission of broadcast messages before periodically transmitting the broadcast messages. Particularly, a BS first transmits a general page message including a broadcast page information in every first slot of a broadcast cycle. Thereafter, a MS can determine the type and time of the message transmission based on the broadcast page and receive broadcast messages in consecutive slots of the broadcast cycle.
A detailed explanation of the periodic broadcast paging, which is most commonly used by communication systems including the CDMA, will follow. However, a description of a common control channel will first be provided. The common control channel is a channel used when a BS transmits an overhead information required for the operation of a MS as well as a general paging message for paging a MS.
Generally, the common control channel is a paging channel or a broadcast channel, but for purposes of explanation, the operation of the common control channel will be discussed with respect to the paging channel. However, such operations can also be performed through a broadcast channel.
FIG. 1 shows a general configuration of a paging channel for the SMS. As shown in FIG. 1, a BS loads and transmits broadcast messages in a slot of 80 ms, where each slot recurs in every 2048 slot of the paging channel with respect to the system time. The slot (SLOT_NUM) at which a broadcast message is transmitted can be calculated by Equation 1 below, where ‘t’ represent a system time for each am of the paging channel.SLOT_NUM=└t/4┘mod2048  [1]
A MS remains in a non-active state and becomes active to receive broadcast messages transmitted in the 80 ms slot of the paging channel. Upon receipt of the messages, the MS returns back to the non-active or idle state. The periodic broadcast paging method in the related art will next be discussed with reference to FIG. 2.
Referring to FIG. 2, a BS transmits broadcast page information in every first slot of a broadcast cycle and transmits broadcast messages in subsequent slots of the broadcast cycle to MSs. A MS which is configured to receive SMS broadcast message then receives the broadcast messages based on broadcast page(s) by periodically monitoring every first slot of the broadcast cycle to receive the broadcast page information. Here, the maximum interval of the broadcast cycle (M) can be calculated by Equation 2, where ‘i’ represents a maximum slot cycle index (MAX_SLOT_CYCLE_INDEX) field value of a system parameter message transmitted to a MS through the paging channel.M=2i×16,0≦i≦7  [2]
Accordingly, based upon the broadcast cycle M, a BS transmits broadcast page information through a first slot of a broadcast cycle, where the slot number is calculated by Equation 3 below. In other words, the BS transmits Broadcast Pages 1, 2, and 3 through the first slot a broadcast cycle, which is indicated as ‘0’ in the paging channel shown in FIG. 2.└t/4┘mod M=0  [3]
Each broadcast page transmitted through the first slot as described above, includes at least one broadcast address (BC_ADDR), where each BC_ADDR includes information regarding a broadcast message. As shown in FIG. 2, the first broadcast page would include two broadcast addresses corresponding to the first broadcast message in slots 3 and 4, the second broadcast page would include a broadcast address corresponding to the second broadcast message in slot 6, and the third broadcast page would include three broadcast addresses corresponding to the third broadcast message in slots 9.about.11. Thus, a BS periodically transmits broadcast page(s) in the first slot and broadcast messages in subsequent slots of the broadcast cycle, after notifying MSs of its broadcast cycle through a system parameter message of the paging channel.
Here, a broadcast index field (BCAST_INDEX) of an expanded system parameter message is used. Namely, a value of BCAST_INDEX is set as ‘i, (1.ltoreq.i.ltoreq.7)’ when the BS is providing a SMS and ‘0’ otherwise. If the value of the BCAST_INDEX is set as ‘i,’ the broadcast cycle in which the broadcast message is transmitted can be calculated using Equation 4.B=2i×16,1≦i≦7  [4]
After the value of B is obtained by Equation 4, the broadcast message is transmitted by a broadcast cycle of (B+3) and the first slot of the broadcast cycle can be calculated by Equation 5.└t/4┘mod M(B+3)=0  [5]
When broadcast messages are periodically transmitted through a paging channel, a MS must also periodically monitor the assigned paging channel. Thus, a MS receives the transmitted broadcast page(s) from the BS through the first slot of the broadcast cycle and if several broadcast addresses (BC_ADDR) are included in the broadcast page(s), the MS checks the subsequent slots to receive the necessary broadcast message.
In the CDMA system of the related art for the SMS, the broadcast index field BCAST_INDEX of the expanded sys parameter message is used only to set the state of a broadcast cycle and thus the SMS on or off. As a result, a BS cannot notify MSs that there is no broadcast message transmitted during certain broadcast cycles. Thus, a MS has to periodically monitor the common control channel to receive broadcast messages from a BS. Accordingly, a MS must periodically check the common control channel for at least every 80 ms, even when there are no messages transmitted from a BS. This unnecessarily wastes the power consumption of a MS.